﻿<p>The <em>IfcFeatureElementSubtraction</em> is specialization of
the general feature element, that represents an existence dependent
elements which modifies the shape and appearance of the associated
master element. The <em>IfcFeatureElementSubtraction</em> offers the
ability to handle shape modifiers as semantic objects within the
IFC object model that subtract from the shape of the master
element.</p>

<blockquote class="note">
NOTE&nbsp; A single subtraction feature
such as the subtype <em>IfcOpeningElement</em> is assigned by a
single subtraction relationship <em>IfcRelVoidsElement</em> to one
occurrences of <em>IfcElement</em>. It establishes a 1:1
relationship between the opening and the element. An element may
have several <em>IfcRelVoidsElement</em> relationships, enabling
several voids.
</blockquote>

<p>The voiding relationship between a master element and a
subtraction feature is geometrically resolved by a Boolean
difference operation.</p>

<p>The local placement for <em>IfcFeatureElementSubtraction</em> is
defined in its supertype <em>IfcProduct</em>. It is defined by the
<em>IfcLocalPlacement</em>, which defines the local coordinate system
that is referenced by all geometric representations. The local
placement is always defined in relation to the local placement of
the building element from which the feature element substration is
substracted:</p>
<ul>
<li>The <em>PlacementRelTo</em> relationship of
<em>IfcLocalPlacement</em> shall point (if given) to the local
placement of the same <em>IfcElement</em>, which is used in the
<em>VoidsElements.RelatingElement</em> inverse attribute.</li>
</ul>

<blockquote class="history">
  HISTORY&nbsp; New entity in IFC2x2.
</blockquote>
<blockquote class="change-ifc2x2">
  IFC2x2 CHANGE&nbsp;  The entity is introduced as an upward compatible extension of the IFC2x
  platform. It is an intermediate abstract supertype without defining its own explicit attributes.
</blockquote>